Our current emphasis in the study of play is investigation of the motivational or social conditions promoting the appearance of playful behavior in squirrel monkeys. Developmental and functional analyses of human play have been the subject of extensive clinical interest, particularly as a symptom and early predictor of psychopathology. Comparable studies of play in animals have been few, although it is commonly assumed that play in animals is an indicator of good mental as well as physical health. Play behavior is almost ubiquitous in young mammals, and is usually the chief means of social interaction between youngsters. Nonetheless, play appears to be quite sensitive to stressors, dropping out of the repertoire, or being greatly curtailed, in response to nutritional or environmental stress, and in some cases to emotional stress, e.g., loss of the mother. In captive animals, particularly juveniles, playfulness is thought to be a sign of general well-being. The experiments are designed to investigate a possible relationship between a physiological measure of stress (plasma cortisol level) and a behavioral assessment of play, using as subjects a population of young squirrel monkeys. Juvenile squirrel monkeys exhibit an extensive repertoire of play behaviors, and their physiological and endocrine responses have been extensively studied, providing a particular favorable opportunity to correlate physiology and behavior. The first hypothesis being investigated is that an inverse relationship exists between an individual monkey's plasma cortisol level and his or her level of play. If a relationship can be established, then additional hypotheses are of Interest: a. Are cortisol levels indicative of non- play behavioral measures of compatibility, such as affiliation, fighting, etc? b. Are the most homogeneous pairs (with respect to gender, age, common experience, etc.) most likely to be compatible and playful? What is the cortisol (stress) profile for pairs that may be compatible but which do not play? We are presently in the initial stages for data acquisition and analysis for 8 young monkeys for whom we have now completed collection of blood samples and behavioral data. The behavioral database from these animals is also being analyzed for finger-grained features of play, including dominance and role reversal, as a means of understanding how partners cooperate to keep play bouts going. Our own prior studies of play in young squirrel monkeys suggest that high rates of play are fostered by equality of dominance. However, in these prior studies, animals always had more than one partner from which to choose.